EPSC Abstracts Vol. 8, EPSC2013-111, 2013 European Planetary Science Congress 2013 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2013

Estimating the volume of glacial ice on Mars: Geographic and geometric constraints on concentric crater fill, lineated valley fill, and lobate debris aprons along the Martian dichotomy boundary

C. Fassett (1), J. Levy (2) and J. Head (3) (1) Mount Holyoke College, South Hadley, MA, USA (2) University of Texas at Austin, USA (3) Brown University, Providence, RI, USA ([email protected])

Abstract Once the spatial extent of these features was established, we are using geometric tools to infer the Landforms inferred to have formed from volumetric extent of glacial landforms. For example, glacial processes are abundant on Mars and include for CCF, several morphometric properties were features such as concentric crater fill (CCF), lobate extracted from catalogues of northern hemisphere debris aprons (LDA), and lineated valley fill (LVF). crater morphologies [13] including: crater diameter, Here, we present new mapping of the spatial extent d and measured crater depth, Dm. CCF fill radius, rf of these landforms derived from CTX and THEMIS is measured from CTX images of the crater, VIS image data, and new geometric constraints on measured along two orthogonal profiles that span the the volume of glaciogenic fill material present in spatial limit of “brain terrain” surface texture or concentric crater fill deposits. concentric surface lineations.

Using these quantities measured from MOLA 1. Introduction and CTX data, relationships between fresh-crater Debris-covered glacial landforms such as depths and diameters on Mars [14], coupled with CCF, LVF, and LDA are widespread on Mars and elementary calculus (solids of rotation), permit us to have been long inferred to represent locations of make quantitative estimates of CCF fill volume (note: abundant ground ice preserved at mid-latitude this method does not distinguish between prior fill locations [1-12]. Here, we present new mapping and and CCF ice-related fill). The functional relationship geometric calculations aimed at determining the between crater diameter (d) and crater depth (D) was volume of non-polar glacial ice on Mars and its quantified by Garvin et al. implications for global martian water budgets. d = wDy (1)

2. Methods where w and y are constants that depend on d (all constants reported by 14 vary as a function of d for LDA, LVF, and CCF were manually mapped simple craters, 1-7 km in diameter, complex craters, using mosaics of CTX and THEMIS VIS image data 7-100 km in diameter, or large craters, >100 km in sampled at 18 m/pixel and viewed at ~1:250k scale diameter). The topographic profile of a crater wall between latitudes 30-50˚ N. Criteria used to identify follows a similar power-law relationship: here, in LDA, CCF, and LVF include the presence of Cartesian coordinates of height (y) and radial parallel/concentric lineations and the presence of n distance (x): y = kx (again, with k and n depending “brain terrain” [12] surface textures or subsequent on d) [14]. These simple geometric relationships mantling on topographically convex near-surface make it possible to calculate the volume of a CCF lobate features (in addition to the range of deposit, assuming an axially symmetric CCF deposit, geomorphic criteria established by [2,7,11]). Results by integrating a solid of rotation defined by of the mapping are shown in Figure 1.

y = kxn (2)

about the y-axis, and integrating from 0 to x = rf. The in volume. Non-glacial ice on Mars represents a volumes of individual CCF deposits were totalled up major ice reservoir on Mars and is a major indicator to calculate the volume of the CCF reservoir in the of paleo-atmospheric conditions. mid-latitude study region, and were transformed into minimum and maximum ice masses, by multiplying 5. References by 30% and 90% ice volume fractions (rock glacier and debris-covered glacier, respectively) and by [1] Sharp, R. P. (1973) JGR, 78, 4073–4083. [2] assuming negligible ice densification from Squyres, S. W. & Carr, M. H. (1986) Science 231, compaction. 249–252. [3] Squyres, S. W. (1978) Icarus 34, 600– 613. [4] Lucchitta, B. K. (1984) LPSC Proceedings 14, B409–B418. [5] Baker, V. R. et al. (1991) Nature 3. Results 352, 589–594. [6] Colaprete, A. & Jakosky, B. M. JGR, 103, 5897–5909. [7] Mangold, N. (2003) JGR, As shown in Figure 1, mapping efforts have 108, doi:10.1029–2002JE001885. [8] Pierce, T. L. & led to a preliminary estimation that glacial landforms Crown, D. A. (2003) Icarus 163, 46–65. [9] Li, H.et cover ~5.8 x 105 km2 of the martian surface between al. (2005) Icarus 176, 382–394. [10] Head, J. W., et 30-50˚. This glacial landform area consists of ~3.5 x al (2010) EPSL, 294, 306–320. [11] Head, J. W., et al 105 km2 of LDA, ~1.6 x 105 km2 of CCF, and ~0.8 x (2006) EPSL, 241, 663–671. [12]Levy, J., et al (2010 105 km2 of LVF. For comparison, the north polar cap Icarus 209, 390–404. [13] Boyce, J. M. (2005) JGR, has an area of ~106 km2 [15]. 110, E03008. [14] Garvin, J. B., et al (2003) 6th Mars, #3277. [15] Zuber, M.T. et al. (1998) Science, Of the 200 largest craters in the northern 282, 2053–2060. [16] Plaut, J. J. et al. (2009) GRL, hemisphere, 100 are located in the study range, and 36, L02203. [17] Holt, J. W. et al. (2008) Science 18 contain CCF deposits. The combined volume of 322, 1235–1238. these crater fill deposits is estimated at ~2 x 103 km3, or 1.8 x 103 km2 assuming a nearly pure ice content [16,17], or 0.6 x 103 km3 of ice, assuming ice only fills CCF pore spaces. This constitutes a global equivalent layer ~1 cm thick. Because many smaller craters are nearly fully-filled with CCF deposits, this CCF volume total is expected to rise dramatically when smaller deposits are measured.

4. Conclusions

Glacial ice deposits on Mars are spatially Figure 2. Schematic illustration of CCF volume extensive, and, through new volumetric calculations. measurements, are being shown to be heterogeneous

Figure 1. Map showing the distribution of CCF, LDA, and LVF based on CTX and THEMIS-VIS mapping in the northern hemisphere. Base map is MOLA elevation. The mapped band spans 30-50˚N.